The present invention relates to radio-frequency (RF) shields and, more particularly, to a novel RF shield for placement between an RF body coil and a set of gradient coils in a nuclear-magnetic resonance (NMR) imaging device.
An NMR imaging device typically utilizes a set of three gradient coils to obtain spatially-selective information. Each of the three gradient coils generally contains a plurality of turns of conductive cable or wire, with integrated lengths of up to several hundred meters. RF fields lose a significant portion of their energy if these fields impinge upon the conductive wires of the gradient coils; while the loss mechanism is not fully understood, it is probably associated with resonances, exciting the gradient structure, having high currents and therefore associated high losses. Any RF power loss, in the gradient coils or otherwise, appears as a lowering of the quality factor Q of the RF coil and consequently appears as a lowering of the signal-to-noise ratio (SNR) attainable in the imaging device. Accordingly, it is highly desirable to prevent penetration of the RF field into the gradient coils; a shield is typically placed between the RF coil and the gradient coils. The RF shield must, however, be substantially transparent to the gradient magnetic fields and therefore must prevent inducement of any significant shield currents at gradient frequencies (typically less than about 10 KHz.) to prevent temporally-dependent and/or spatially-dependent magnetic field inhomogeneities from appearing and having an adverse affect on the resulting image.
Hitherto, RF shields for NMR imaging devices have used a copper-dielectric-copper laminate with an overlapping patchwork pattern etched into the copper sheets on both sides of the laminate. The patchwork patterns have generally been poor approximations to the current paths in a solid shield, so that induced currents are forced to flow through the shield dielectric at several locations. Thus, the current path contains the equivalent of several capacitors in series, and, undesirably, total path capacitance is relatively small. It is desirable to not only provide a highly effective RF shield for placement between an RF coil and a set of gradients coils, but also to provide such an RF coil in which the shield current flows through at most a single shield capacitor. It is also desirable to reduce the cost of an RF shield by removing the necessity for a conductor-dielectric-conductor laminate and utilizing a single conductive sheet.